The present invention is an improvement to structured packing used in liquid-vapor contact columns and to columns using structured packing. The improvement comprises occluding peripheral openings of the structured packing that open into an interspace formed between the column shell and the structured packing when they are positioned in non-occlusive opposition to each other with a barrier film. The occlusion of the peripheral openings reduces vapor flow in the interspace thereby improving liquid-vapor contact in the structured packing and improving column performance.
Structured packing is being increasingly utilized in the process industry in liquid-vapor contact columns. Structured packing offers the unique advantages of increased efficiency, lower pressure drop, greater capacity and net energy savings when compared to conventional heat and mass transfer devices such as random packed columns and tray columns. Because of these advantages structured packing is increasingly being used to retrofit existing separation columns.
Structured packings typically are fairly rigid pre-formed structures comprising ordered elements which form channels opening to the periphery of the packing. The materials of construction for such structured packings can include corrugated meshes and sheets fabricated from, for example, metals, plastics, and ceramics. Therefore, the structured packings may not easily be conformed to the inner wall of a column shell and in the situation where the inner wall of a column is non-uniform it may be necessary to undersize the pre-formed structured packing to facilitate its placement in the column. In these situations the channels opening to the periphery of the structured packing provide a means for vapors to enter into the interspace between the column shell and the structured packing, thereby reducing contact of the vapor phase with the liquid phase contained in the structured packing. This vapor by-passing produces a discontinuity in liquid-vapor contact in the column reducing heat and mass transfer between vapors and liquids in the column and results in reduced column performance.
One attempt to reduce this discontinuity in liquid-vapor contact has been to fit the-structured packing with one or more bands on the outside of the packing elements that serve as "wipers" and are intended to provide a seal between the packing elements and the column shell. The use of such wiper bands is described, for example, in McKeigue et al., U.S. Pat. No. 5,486,318. Jeannot et. al., U.S. Pat. No. 5,224,351, describe a sealing device prepared from a continuous band of an appropriate material of small thickness such as a metal or plastic.
The present inventors have discovered that in those situations where an interspace exist between the column shell and the structured packing positioned therein that such bands are not entirely satisfactory because significant vapor flow can by-pass the structured packing by flowing through the interspace between the bands. The present invention is intended to reduce the vapor exiting the structured packings through the laterally orientated peripherally opening channels and therefore reduce the flow of vapors exterior to the bulk of the structured packing.
Another method that has been proposed to reduce this discontinuity in liquid-vapor contact is to fill the interspace space between the column shell and the structured packing with a random packing. Using this method is often difficult in columns having irregular surfaces to insure adequate filling of the space to avoid channeling and subsequent removal of the structured packing is made more difficult and time consuming.